<p>Urban meteorology has developed in parallel to other
sub-fields in the science, but in many ways remains poorly described. In
particular, the study of urban rainfall modification remains behind compared to
other comparable features. Urban rainfall modification refers to the change of
a precipitation feature as it crosses an urban area. Typically, this manifests
as rainfall initiation, local suppression, local invigoration, and/or storm
morphology changes. Research in the prior decades have shown urban rainfall
modification to arise from a combination of land-atmosphere and aerosol-cloud
interaction. Urban areas create a greater surface roughness, which produces
local convergence and divergence, modifying local thunderstorm inflow and
morphology. The land surface also generates vertical velocity perturbations
which can act to initiate or modify existing convection. Urban aerosols act as
CCN to perturb existing cloud and precipitation characteristics. Higher CCN
narrows the cloud droplet distribution, creating more smaller cloud droplets,
and initially reducing precipitation efficiency by keeping more liquid water in
the cloud than what would form into rain. The CCN-cloud interaction eventually
increasing heavy rainfall production as graupel riming is enhanced by the
narrower cloud droplet distribution, leading to more larger raindrops and
higher rain in areas.</p><p>This dissertation addresses the observation and modeling of
urban thunderstorm interaction from both the land surface and aerosol
perspective. It reassesses the original urban rainfall anomaly: The La Porte
Anomaly. First analyzed in the late 1960s, the La Porte Anomaly was ultimately
dismissed by 1980 as either a temporary, biased, or otherwise unexplainable
observation, as the process level understanding had yet to be explained. The
contemporary analysis utilizes all existing data and objective optimal
interpolation to show that a rainfall anomaly downwind of Chicago has indeed
existed at least since the 1930s. The current rainfall anomaly exists as a
broad region of warm season rainfall downwind of Chicago that is 20-30% greater
than the regional average. Using synoptic parameters, the rainfall anomaly is
shown to be independent of wind direction and most closely associated with
local land surface forcing. Weekdays, where local aerosol loading has been
measured at 40% or more greater than weekends, have up to 50% more warm season
rainfall than weekends. The analysis is able to show that there is a land
surface and aerosol contribution to the rainfall anomaly, but cannot
unambiguously separate them.</p><p>In order to separate the land
surface and aerosol effects on urban rainfall distribution, a numerical model
was improved to better handle urban weather interaction. The Regional
Atmospheric Modeling System (RAMS 6.0) was chosen for its base land surface and
cloud physics parameterization. The Town Energy Budget (TEB) urban canopy model
was coupled to RAMS to handle the urban land surface. The Simple Photochemical
Module (SPM) was coupled with the cloud physics to handle conversion of surface
emissions to CCN. The model utilized an external traffic simulation to create a
realistic diurnal and weekly cycle of surface emissions, based on human
behavior. The new Urban RAMS was used to study the land surface sensitivity of
city size and of aerosol loading in two studies using the Real Atmosphere
Idealized Land surface (RAIL) method, by which all non-urban features of the
land surface are removed to isolate the urban effects. The city size study
determined that the land surface of a given city eventually has a maximum
effect on thunderstorm modifying potential, and that rainfall does not continue
to increase or decrease locally for cities larger than a certain size based on
that storm’s own motion. The aerosol-cloud analysis corroborated previous
observations on the non-linear effects of aerosol loading on clouds. It also
demonstrated that understanding the aerosol effect in an urban environment
requires high resolution observations of precipitation change. In a single
thunderstorm, regions can be both impacted by local rainfall rate increases and
decreases from urban aerosols, leading to little total change in precipitation.
But the rainfall rate changes can significantly affect soil moisture and
drought potential in and around urban areas.Following the idealized studies,
the historical and current La Porte Anomaly was simulated to separate the land
surface from the aerosol factors near the Chicago area. The Urban RAMS model
was deployed on a real land surface with full model physics. Simulations with
1932, 1962, 1992, and 2012 land covers were run over an exceptionally wet Aug.
2007 to approximate the rain variability for an entire summer season. Surface
emissions were also varied in the 2012 land cover for variable aerosol loading.
The simulations successfully reproduced the location of the downwind rainfall
anomaly in each land cover scenario: farther east toward La Porte in 1932,
moving southwestward to its current location by 2012. Doubling surface
emissions eliminated the downwind anomaly, as was observed during the highest
pollution decade of the 1970s. Eliminating surface emissions also decreased the
downwind anomaly. As the land cover at the upwind edge of Chicago became more
connected from the 1932 to 2012 land cover scenarios, a local upwind rainfall
anomaly developed, moving westward with urban expansion. The results of these
simulations enabled the conclusions that a) at the upwind edge, the land
surface dominates urban rainfall modification, b) the aerosol loading sustains
and increases the locally downwind rainfall increase, and c) that the total
modification distance is static on given day and given urban footprint. A more
expansive city does not produce a rainfall anomaly more distantly downwind, but
rather the distance of rainfall modification moves to where the upwind edge of
the city begins.</p><p></p><p>The modeling work ends with a
two-city simulation in the southeast United States, of a bow-echo forming near
Memphis, TN and crossing Birmingham, AL before splitting. Simulations were
performed on different surface emissions rates, land covers where Birmingham
did not exist, and a novel approach with two inner emitting grids over both
Birmingham and Memphis. A storm tracking algorithm enabled one-to-one
comparisons of point simulated storm characteristics between scenarios. The
results of most scenarios only corroborated previous research, showing how
increased aerosol loading changes cloud and rainfall characteristics until the
highest aerosol loading shuts down riming and rainfall enhancement. However, the
two most accurate simulations, where the storm forms and splits over
Birmingham, were a non-urban higher rural aerosol scenario and the scenario
with Memphis also emitting pollution. In order to split the storm over
Birmingham, the upwind cloud characteristics were primed by higher upwind
aerosols, either from a realistic city upwind or unrealistically high rural
aerosols. The conclusions produced by this study demonstrated the importance of
aerosol cloud interaction, perhaps equal with land surface, but also the need
for far upwind information for a storm in a given city. Memphis and Birmingham
are separated by over 300km, far exceeding the threshold thought to connect two
cities by mutual rainfall modification.</p><p>The overall conclusions of the research presented in this dissertation shows a more unified approach to the effects of urban rainfall modification. The upwind edge of a city is a fixed location, and a thunderstorm begins modifying at that point. The thunderstorm usually produces a local rainfall maximum at the upwind edge, due to the vertical velocity of the urban land surface. The urban aerosols proceed to narrow the cloud droplet distribution, locally reducing rainfall as the storm passes over the urban area. Eventually the enhanced rainfall from enhanced riming produces a maximum somewhere downwind. However, “downwind” is a location relative to the storm’s motion and could exist anywhere over the urban footprint or downwind in a rural region. The climatological location of increased rainfall is an average of every storm in a season and beyond. The results of each part of the study provide a way to continue the research presented here.</p><br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12275627 |
| Date | 12 May 2020 |
| Creators | Paul E. Schmid (5930237) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Observing_and_Modeling_Urban_Thunderstorm_Modification_Due_to_Land_Surface_and_Aerosol_Effects/12275627 |
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